A conventional DC to DC converter circuit, such as a step-down synchronous rectification switching converter, acquires a DC voltage, different from a DC power having a substantially constant voltage, by switching a transistor element. In such a DC to DC converter circuit, for example, when providing overcurrent protection for a load or detecting a light load state having a small output current and realizing a function of automatically shifting to a light load mode, an average current actually supplied to the load must be detected, as disclosed in JP-A-2005-65447 and US 2005/0057229 A1.
FIG. 4 is a circuit diagram showing a current detecting device 90 of a conventional DC to DC converter. The current detecting device has a control circuit 91, which is connected to a switching circuit 92, that lowers and controls the power supply voltage of a DC input power source VDD to a predetermined DC voltage. The switching circuit 92 forming an output stage includes a high-side switch SW91 and a low-side switch SW92. The high-side switch SW91 is supplied with the DC input power VDD from one end and has the other end connected to one end of the low-side switch SW92. The low-side switch SW92 has the other end grounded. A load 94 is connected to the switching circuit 92 via a smoothing circuit 93, which includes an inductor L91 and an output capacitor C91. As the pair of switches SW91 and SW92 are alternately turned on and off at the timing determined by the control circuit 91, an output voltage VO90 of predetermined magnitude is supplied to the load 94. The synchronous rectification DC to DC converter is thus constructed. A low-pass filter is provided between a node M90 of the switching circuit 92 and the smoothing circuit 93, and a detection terminal 96. This low-pass filter 95 includes a filter resistor RLP91 having one end connected to the node M90 and having the other end connected to the detection terminal 96, and a filter capacitor CLP91 having one end connected to the other end side of the filter resistor RLP91 and having the other end grounded.
In the switching circuit 92 of FIG. 4, each of the pair of switches SW91 and SW92 has a switching transistor, such as a MOSFET, with a parasitic resistance component. In FIG. 4, the on-resistance components of the switches SW91 and SW92 are explicitly shown as Rhi, Rlow. Diodes D91 and D92 are connected parallel to the respective switches SW91 and SW92. The diodes D91 and D92 can be parasitic diodes generated when a MOS transistor is used as the switching transistor.
The current detecting device 90 detects an average value of a current flowing through the inductor L91 (i.e., output current) based on the difference between a voltage Vma output to the detection terminal 96 of the current detecting device 90 and the output voltage VO90 being equal to an average value of voltage drop due to a parasitic series resistance Rind91 of the inductor L91. This current detecting device 90 can advantageously avoid efficiency loss caused by power loss of the detection resistance, in comparison with the case where the current detection resistance is connected in series with the inductor.
However, the value of the parasitic series resistance Rind91 of the inductor L91 is small. Particularly, when the device is used for detecting a light load state, the difference between the voltage Vma and the output voltage VO90 is several mV or less. Therefore, when comparing the two voltages by an ordinary amplifier, a large error can occur because of the variation in the input offset voltage, thus making such a device unpractical.
Accordingly, there remains a need to provide a voltage detecting circuit and a current detecting circuit that can detect an output current with high accuracy without lowering the efficiency by detection resistance. The present invention addresses this need.